In the electrophotographic art, it is well known to use a reusable photoconductor for the production of copies on plain paper, and typically the photoconductor may take one of two forms. In one form, the surface of a drum is suitably coated with a photoconductive material and as the drum rotates within the photocopy machine, an image of an original document is projected onto the photoconductive material by any of a number of well known scanning techniques. The drum rotates past a series of circumferentially arranged operating stations, at the first of which the photoconductive material is uniformly electrostatically charged. The second station is the exposing station where the photoconductive material is exposed to an optical image of the original document, and the photoconductive material is discharged in the light-struck areas in a manner well known in the art. The drum rotates to bring the imaged area of the photoconductive material past a developing station at which developing material is applied to the photoconductive material to render the image visible; both the developing material and several techniques for applying it are well known in the art. Thereafter, the drum rotates to bring the developed image into contact with copy paper at a transfer station to which the copy paper is fed in synchronism with the rotation of the drum, after which the toner image is fixed on the copy paper by well known techniques. The drum continues to rotate to bring the imaged area of the photoconductive material to a cleaning station at which any residual developing material not transferred to the copy paper is cleaned from the photoconductive material. The entire foregoing cycle of operation then repeats itself for as many copies of the document as are desired.
One of the principal disadvantages of the drum type photoconductor is the requirement that the image of the original document be projected onto the photoconductive surface by an optical scanning technique. This necessitates either synchronized movement of the original document and the photoconductor drum through a stationary illumination and optical system or synchronized movement of the photoconductor drum with a movable illumination and optical system while maintaining the original document stationary. Both of these systems are relatively complicated and expensive in terms of hardware and, together with service and maintenance disadvantages, are not desirable. Also, the speed of operation of the copying machine is seriously limited by the time required to scan an original document with the result that many optical scanning type photocopy machines are relatively slow.
These disadvantages are partially overcome by presently known photocopy machines which utilize the so-called full frame or flash exposure imaging technique. Under this technique, a photoconductive material applied to a suitable supporting member is maintained in a flat planar configuration, and an image of the original document is projected onto the photoconductive material by subjecting the document to a very high intensity flash of light for an extremely short interval, much like a typical flash exposure in conventional photography. The light intensity is sufficiently high and the duration sufficiently short that the photoconductive material can be exposed even while it is moving and still result in a clear sharp image. Since this technique is so rapid, all scanning mechanism can be eliminated in a photocopy machine utilizing this technique and the speed of operation considerably increased.
It will be recognized that photoconductive materials suitable for this technique have inherent problems in maintaining the portion being imaged in a flat condition. Obviously, the photoconductive material must be confined within the limits of the photocopy machine and to repetitively make copies at high speeds the photoconductive material must travel in a closed loop path in order to pass through the required operating stations described above repeatedly. Typically, the photoconductive material is suitably carried by a flexible substrate which can be moved along a circuitous path which includes a straight portion which defines the imaging or exposing location of the photoconductive material.
One way to accomplish this is to provide the flexible substrate in the form of an endless belt coated with photoconductive material, the belt passing around suitable guides mounted in the photocopy machine which define both a flat planar section of the belt path for exposing the photoconductive material and the remaining portion of the belt path, which may take any suitable configuration, along which the other processing stations are appropriately spaced. A typical closed loop belt arrangement is shown in U.S. Pat. No. 3,661,452 to Hewes et al. One major difficulty with this arrangement is that it is only practical with selenium as the photoconductive material and precludes the use of other photoconductive materials. This is so for the reason that commercially available inorganic photoconductive materials such as zinc oxide are too slow in adapting to the dark after having been exposed to light to accept a charge within the short time that the belt takes to make one complete excursion along the belt path. The belt path would have to be immensely long in order to store any given segment of the photoconductive material time to adapt to the dark. Organic photoconductive materials are also commercially available, and these materials adapt to the dark and become rechargeable at a very rapid rate, thereby eliminating the drawback of the inorganic photoconductive materials to closed loop belt use. However, organic photoconductive materials are notably weak and subject to abrasion from the usual forms of photoconductor cleaners and accordingly must be replaced considerably more frequently than inorganic photoconductors and far more frequently than selenium photoconductors. Again, therefore in order to provide a photoconductive element which will yield a commercially acceptable number of exposures, a substantially long length of photoconductive material must be provided; far in excess of that which could be provided in a closed loop belt configuration.
One technique for utilizing a long indefinite length of photoconductive material without encountering the severe storage problem of a long closed loop belt is shown in U.S. Pat. No. 3,588,242 to Berlier. A rotatably mounted hollow drum has a supply spool of organic photoconductor mounted therein as well as a take up spool and the photoconductor passes from the supply spool through a slit in the drum surface and thence around the peripheral surface of the drum and back through the slit to the take-up spool. In operation, the drum rotates and copies are made exactly as described above with respect to the selenium coated photoconductor drum. When the segment of photoconductive material around the drum becomes unusuable for the production of satisfactory copies, a drive mechanism operates to rotate the take-up spool sufficiently to place a fresh segment of photoconductor around the drum. When the entire photoconductor is used, it is replaced with a fresh supply spool. It will be apparent that this technique requires an optical scan imaging technique of one of the types described above and therefore inherently possesses all of the disadvantages previously described which are attendant to this technique.
Earlier attempts to avoid both the optical scanning technique and the belt photoconductor arrangement and the disadvantages and limitations attendant thereto are illustrated by U.S. Pat. Nos. 3,584,947 and 3,706,489. In the earlier patent, a plurality of individual photoconductive plates are attached to a framework which is carried by a rotatably mounted drum. As the drum rotates, the framework operates through a linkage mechanism to maintain the photoconductive plate in a planar configuration so as to permit full frame flash exposure, after which the framework operates to cause the photoconductive plate to lie upon and conform to the curvature of the rotating drum to facilitate the operation of the other functions of the copying machine. It will be apparent that the enormous complexity of the framework and the actuating mechanism therefor, as well as the limited life of the photoconductive plates, present serious drawbacks to a copying machine of this type becoming commercially successful.
In the later patent a supply roll of a web photoconductor and a take-up spool are both mounted inside of a rotatably mounted drum, and the photoconductor passes over a flat bed or imaging plate mounted on the drum. In operation, the portion of the photoconductor overlying the flat plate is imaged by flash exposure, and the web is then moved to bring the exposed portion to a position where it lies on a curved portion of the drum so that the other above described operations of a copying cycle can be performed during rotation of the drum. In moving the web as just described, a fresh portion is brought into position across the flat bed for the next copying cycle. It will be apparent that this arrangement presents a very inefficient use of the photoconductor web since each segment which is exposed is used once to make a copy and is then wound on the take-up spool, except for the case of making multiple copies of the same document. Even then, the photoconductor cannot be re-exposed for the same document on the same section of photoconductor, and multiple copies will become progressively poorer in quality due to gradual degradation of the charge image on the photoconductor with each developing and transfer operation.
It is apparent from the foregoing that, prior to the present invention, there is no commercially satisfactory arrangement for handling a web photoconductor which avoids all the disadvantages of prior art techniques and yet retains the benefits found individually in each technique.